{"gene":"RAB18","run_date":"2026-06-10T06:43:36","timeline":{"discoveries":[{"year":2005,"finding":"Rab18 localizes to lipid droplets (LDs) in HepG2 cells; wild-type and dominant-active Rab18 localize to LDs but the dominant-negative form does not. Rab18 overexpression induces close apposition of LDs to rough ER-derived membrane cisternae (termed LD-associated membrane, LAM), and this is linked to a decrease in ADRP on LDs.","method":"Proteomic analysis of LD fraction, immunohistochemistry, western blotting, dominant-active/dominant-negative mutant expression, RNA interference, brefeldin A treatment","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (proteomics, IHC, dominant mutants, RNAi, BFA), replicated across cell lines","pmids":["15914536"],"is_preprint":false},{"year":2005,"finding":"GFP-Rab18 localizes directly to the phospholipid monolayer surface of LDs (shown by immunoelectron microscopy), and this association is increased upon lipolytic stimulation of adipocytes by beta-adrenergic agonists. A caveolin-3 truncation mutant that inhibits LD catabolism labels mutually exclusive LD subpopulations from Rab18-labeled LDs.","method":"GFP tagging, immunoelectron microscopy, lipolytic stimulation, beta-adrenergic antagonist reversal, co-expression of caveolin-3 mutant","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — immunoelectron microscopy directly demonstrates monolayer surface association; functional regulation by lipolytic stimulation shown with antagonist reversal","pmids":["16207721"],"is_preprint":false},{"year":2007,"finding":"In neuroendocrine cells (PC12 and AtT20), Rab18 associates with a subpopulation of secretory granules upon stimulation of the regulated secretory pathway, and this association is linked to inhibition of secretory activity. Dominant-active Rab18 associates with secretory granules; dominant-inactive Rab18 remains cytosolic.","method":"Immunofluorescence, immunoelectron microscopy, dominant-active/dominant-negative mutant expression, secretion assays in PC12 and AtT20 cells","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 2 / Moderate — immunoelectron microscopy plus functional secretion assays with GTPase mutants in two cell lines","pmids":["17488286"],"is_preprint":false},{"year":2008,"finding":"Rab18 overexpression or siRNA knockdown severely disrupts the Golgi complex and reduces secretion of VSVG. Rab18 mutants specifically enhance retrograde COPI-independent Golgi-to-ER transport of beta-1,4-galactosyltransferase-YFP but not COPI-dependent p58-YFP cargo. Rab18-S22N potentiates BFA-induced ER-Golgi fusion, indicating a role for Rab18 in ER-Golgi trafficking.","method":"siRNA knockdown, dominant-negative mutant expression, VSVG-GFP secretion assay, photobleach assay for retrograde Golgi-ER transport, BFA treatment","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (siRNA, dominant mutants, live-cell transport assays) showing Golgi disruption and selective retrograde trafficking defects","pmids":["18664496"],"is_preprint":false},{"year":2011,"finding":"Loss-of-function mutations in RAB18 (Leu24Gln and Arg93del) cause Warburg Micro syndrome. Nucleotide binding assays showed these mutant proteins are functionally null — unable to bind guanine nucleotides. Knockdown of rab18 in zebrafish confirmed a conserved developmental role.","method":"Autozygosity mapping, Sanger sequencing, MLPA, nucleotide binding assays, zebrafish rab18 knockdown","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — direct biochemical assay (nucleotide binding) demonstrating functional null mutations, combined with genetic mapping and in vivo knockdown","pmids":["21473985"],"is_preprint":false},{"year":2011,"finding":"Insulin induces Rab18 recruitment to LD surfaces via phosphatidylinositol 3-kinase (PI3K) signaling in 3T3-L1 adipocytes. Rab18 overexpression increases basal lipogenesis, and Rab18 silencing impairs the lipogenic response to insulin, establishing Rab18 as a mediator of insulin-stimulated lipogenesis as well as beta-adrenergic lipolysis.","method":"PI3K inhibitor treatment, Rab18 overexpression, siRNA silencing, lipogenesis assays in 3T3-L1 adipocytes","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — PI3K inhibitor and genetic gain/loss-of-function in adipocytes with functional lipogenesis readout, single lab","pmids":["21829560"],"is_preprint":false},{"year":2013,"finding":"Rab18 binds to the HCV nonstructural protein NS5A; GTP-bound (active) Rab18 binds NS5A more strongly than constitutively GDP-bound Rab18. Rab18 promotes physical association of NS5A and other replicase components with LDs in HCV-infected cells and modulates HCV genome replication and production of infectious virions.","method":"Unbiased proteomics, co-immunoprecipitation, GTP/GDP-locked Rab18 mutant binding assays, colocalization studies in HCV-infected cells, Rab18 knockdown/overexpression with viral output measurement","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP with GTPase state-dependent binding, colocalization, and functional knockdown/overexpression with viral phenotype","pmids":["23935497"],"is_preprint":false},{"year":2014,"finding":"The Rab3GAP complex (RAB3GAP1/RAB3GAP2 heterodimer) is a specific guanine nucleotide exchange factor (GEF) for Rab18. Rab3GAP localizes to the ER and is necessary and sufficient for ER targeting of Rab18. In the absence of Rab3GAP or Rab18 function, ER tubular networks (marked by reticulon 4) are disrupted and ER sheets (CLIMP-63) spread into the cell periphery. Disease-associated point mutations in Rab3GAP1 (T18P, E24V) and Rab3GAP2 (R426C) abolish GEF and membrane-targeting activities.","method":"GEF activity assay, subcellular fractionation, immunofluorescence with ER markers, dominant-negative and disease mutants, siRNA knockdown, Rab18 membrane recruitment assay","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — direct GEF activity assay plus multiple orthogonal functional readouts (ER morphology, membrane targeting) and disease mutant validation","pmids":["24891604"],"is_preprint":false},{"year":2014,"finding":"Active (GTP-bound) Rab18 is required for targeting of fatty acid synthase (FASN) to ER and LD sites of dengue virus (DENV) replication. The interaction of FASN with DENV NS3 protein depends on conversion of Rab18 from GDP- to GTP-bound form. Inactive Rab18 or Rab18 knockdown reduces DENV replication, viral protein/RNA production, and LD induction.","method":"GTP/GDP-locked Rab18 mutant expression, siRNA knockdown, colocalization, co-immunoprecipitation, viral output measurement (RNA, protein, progeny)","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP, GTPase state-dependent FASN recruitment, functional knockdown and mutant overexpression with viral phenotype, multiple readouts","pmids":["24696471"],"is_preprint":false},{"year":2014,"finding":"Rab18 is required for trafficking of the HCV core protein to LDs for viral assembly. Rab18 knockdown reduces intracellular and extracellular viral infectivity but not intracellular viral replication, and blocks recruitment of the HCV core protein to LDs. Rab18 overexpression increases extracellular viral infectivity ~2-fold.","method":"Rab18 siRNA knockdown, Rab18 overexpression, colocalization of Rab18 with core protein, viral infectivity assays","journal":"Virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — complementary knockdown/overexpression with functional readouts and colocalization, single lab","pmids":["24997429"],"is_preprint":false},{"year":2015,"finding":"TBC1D20 shows RAB18 GTPase-activating protein (GAP) activity in vitro and is a physiological substrate regulator of RAB18. In cells lacking TBC1D20, RAB18 becomes more stably ER-associated and less cytosolic, indicating TBC1D20 promotes RAB18 cycling off the ER membrane. This establishes RAB18 as a substrate of TBC1D20.","method":"In vitro GAP activity assay, subcellular fractionation, immunofluorescence of RAB18 localization in TBC1D20-null cells","journal":"Open biology","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — in vitro GAP assay plus cellular localization data, but modest GAP activity noted in vitro; single lab","pmids":["26063829"],"is_preprint":false},{"year":2015,"finding":"In hepatic stellate cells, Rab18 GTPase activity and isoprenylation are required for stellate cell LD loss and activation marker induction. Rab18 knockdown retards LD loss and blocks activation; Rab18 mRNA and protein increase during activation. Rab18 is identified as a retinoic acid-responsive, LD-associated protein in HSCs.","method":"siRNA knockdown, plasmid overexpression, functional mutants (GTPase-dead and isoprenylation-defective), gene array, immunocytochemistry, in vivo liver injury model","journal":"Hepatology (Baltimore, Md.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional mutants and knockdown with cellular phenotype readout in primary and transformed cells, single lab","pmids":["25482505"],"is_preprint":false},{"year":2016,"finding":"Mammalian TRAPPII complex acts as a GEF for both Rab18 and Rab1. Inactivation of TRAPPII-specific subunits (siRNA or CRISPR-Cas9 deletion) impairs Rab18 recruitment to LD surfaces and results in aberrantly large LDs with reduced lipolysis. The COPI-TRAPPII interaction is required for Rab18 recruitment to LDs, suggesting COPI brings TRAPPII to the LD surface where TRAPPII activates Rab18.","method":"GEF activity assay, siRNA depletion, CRISPR-Cas9 deletion, LD morphology analysis, Rab18 localization assay","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — direct GEF assay combined with CRISPR knockout and siRNA, multiple orthogonal methods demonstrating Rab18 activation and LD recruitment","pmids":["28003315"],"is_preprint":false},{"year":2016,"finding":"In developing mouse cerebral cortex, RAB18 regulates neuronal radial migration and morphogenesis. Suppression of RAB18 by in utero electroporation, or overexpression of dominant-negative RAB18, impairs radial migration. Loss of RAB18 accelerates N-cadherin degradation via the lysosomal pathway, reducing surface N-cadherin levels on neurons.","method":"In utero electroporation (shRNA knockdown and dominant-negative overexpression), in vitro neurite growth assays, N-cadherin surface level measurement, lysosomal inhibitor treatment","journal":"Molecular brain","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo electroporation with migration phenotype and mechanistic follow-up showing N-cadherin lysosomal degradation, single lab","pmids":["26879639"],"is_preprint":false},{"year":2017,"finding":"RAB18 positively modulates macroautophagy in primary human fibroblasts; knockdown reduces autophagic activity while overexpression enhances it. This function depends on RAB3GAP1 and RAB3GAP2 as GEFs. RAB18 knockdown also impairs proteostasis, causing accumulation of ubiquitinated proteins.","method":"siRNA knockdown, overexpression, autophagy flux assays, ubiquitinated protein accumulation measurement, RAB3GAP1/2 co-knockdown epistasis","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with RAB3GAP1/2 plus gain/loss-of-function autophagy assays, single lab","pmids":["28342870"],"is_preprint":false},{"year":2017,"finding":"Rab18 is required for BK polyomavirus (BKPyV) infection; genome-wide siRNA screen and follow-up identified Rab18 and syntaxin 18 as essential host factors. Rab18 and the NRZ complex function in retrograde transport of BKPyV from late endosomes to the ER lumen.","method":"Whole-genome siRNA screen, siRNA knockdown validation, viral infection assays in primary human renal proximal tubule epithelial cells","journal":"mSphere","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genome-wide screen followed by targeted validation, functional viral infectivity readout, single lab","pmids":["28815213"],"is_preprint":false},{"year":2018,"finding":"Rab18 controls LD growth and maturation by tethering the ER to LDs. LD-associated Rab18 (activated by its GEF Rab3GAP1/2) binds specifically to the ER-associated NRZ (NAG-RINT1-ZW10) tethering complex and associated SNAREs (Syntaxin18, Use1, BNIP1), recruiting ER to LDs and forming direct ER-LD contacts. Rab18 deficiency or NRZ/SNARE defects cause drastically reduced mature LD numbers, decreased lipid storage, and increased ER stress.","method":"Co-immunoprecipitation, pulldown assays, CRISPR/siRNA knockdown of Rab18 and NRZ/SNARE components, LD morphology and lipid quantification, ER stress markers","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — biochemical pulldown/co-IP identifying the Rab18-NRZ-SNARE complex plus functional rescue/knockdown experiments with multiple orthogonal readouts","pmids":["29367353"],"is_preprint":false},{"year":2018,"finding":"In human mammary carcinoma cells, Rab18 localizes to the ER and LD membranes in an activation-state-dependent manner, but knockout of Rab18 causes only modest reductions in LD size and numbers with no evidence for essential roles in LD formation, LD turnover on starvation, or targeting of several LD proteins.","method":"Rab18 knockout (CRISPR), LD morphology quantification, protein targeting assays, starvation-induced LD turnover","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR knockout with multiple negative functional readouts; this represents a cell-type-specific negative result in mammary carcinoma cells","pmids":["29949452"],"is_preprint":false},{"year":2019,"finding":"RAB18 knockout impairs lipid droplet catabolism and fatty acid release, reducing lipid availability for autophagosome formation. Compensatory adaptations include increased ATG2B expression and phosphorylation, increased ATG12-ATG5 conjugate formation, and enhanced ATG9A phosphorylation at Y8 and S14, which increases ATG9A trafficking. Y8 phosphorylation of ATG9A is important to maintain basal autophagy under RAB18 knockout conditions.","method":"RAB18 knockout, autophagy flux assays, lipidomics, phosphoproteomics, pharmacological inhibition of Y8 phosphorylation, ATG9A trafficking assays","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO + pharmacological inhibition with phosphoproteomic mechanistic follow-up, single lab","pmids":["31874152"],"is_preprint":false},{"year":2019,"finding":"In neurons, Rab18 associates with lysosomes, actively traffics along neurites, and colocalizes/cofractionates/coprecipitates with Rab7. Rab18-deficient neurons show impaired lysosomal transport and aberrant autophagy. Rab7 is upregulated in Rab18-deficient neurons, suggesting compensatory regulation between Rab18 and Rab7 in lysosomal/autophagic activities.","method":"Immunofluorescence, subcellular fractionation, co-immunoprecipitation, live-cell imaging of lysosomal transport, autophagosome marker assays, Rab18-/- mouse model, electron microscopy","journal":"Molecular neurobiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP and fractionation plus functional KO phenotype in neurons with multiple readouts, single lab","pmids":["30721447"],"is_preprint":false},{"year":2020,"finding":"GTP-bound Rab18 (in Drosophila) binds Atg6/Beclin1, a permanent subunit of Vps34 complexes. The Rab3GAP-Rab18 module promotes autolysosome maturation through interaction with Vps34 Complex I. Loss of Rab3GAP2 or Rab18 destabilizes Rab7-positive compartments and perturbs lysosomal biosynthetic transport. Rab3GAP2 and Rab18 are present on autophagosomal and autolysosomal membranes, colocalizing with Vps34 Complex I subunits.","method":"Co-immunoprecipitation (GTP-bound Rab18 pull-down with Atg6/Beclin1), genetic epistasis (Atg14/UVRAG overexpression phenocopying Rab3GAP-Rab18 loss), Drosophila mutant model, colocalization studies","journal":"The FEBS journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP with GTPase state specificity plus epistasis in Drosophila model, single lab","pmids":["32248620"],"is_preprint":false},{"year":2020,"finding":"Rab18 interacts with the ER-resident protein kinectin-1 and controls anterograde kinesin-1-dependent ER transport required for maturation of nascent focal adhesions. Rab18 knockdown reduces focal adhesion size and impairs directional migration in chemotaxis. Rab18 promotes ER-focal adhesion contact formation.","method":"siRNA knockdown, live-cell TIRF microscopy of focal adhesions, co-immunoprecipitation of Rab18 with kinectin-1, ER transport assays, chemotaxis assays","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct co-IP with kinectin-1, live imaging of FA dynamics, functional migration readout, multiple orthogonal methods in single lab","pmids":["32525992"],"is_preprint":false},{"year":2020,"finding":"Rab18 binds to classical swine fever virus (CSFV) NS5A protein; co-immunoprecipitation, GST-pulldown, and confocal microscopy confirm the interaction. Rab18 knockdown reduces CSFV RNA replication and virion assembly, and NS5A is redistributed in Rab18-knockdown cells.","method":"Co-immunoprecipitation, GST-pulldown, confocal microscopy, siRNA knockdown, RT-qPCR, IFA for viral protein localization","journal":"Virulence","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP plus GST-pulldown confirming direct binding, and functional knockdown/overexpression with viral readouts, single lab","pmids":["32419589"],"is_preprint":false},{"year":2021,"finding":"LD-associated Rab18 binds to the C-terminus of perilipin 2 (PLIN2); PLIN2 depletion diminishes LD localization of Rab18. Rab18 also interacts with ACSL3 and promotes ACSL3 LD localization, forming a Rab18-PLIN2-ACSL3 complex. Loss of Rab18 reduces triacylglycerol levels and causes fewer but larger LDs in myoblast cells; overexpression increases TAG and LD number.","method":"Co-immunoprecipitation, siRNA knockdown of PLIN2 and ACSL3, Rab18 overexpression/knockdown, TAG quantification, LD morphology analysis in C2C12 myoblast cells","journal":"Biochimica et biophysica acta. Molecular and cell biology of lipids","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP identifying PLIN2 and ACSL3 as binding partners with functional LD/TAG readouts, single lab","pmids":["33713834"],"is_preprint":false},{"year":2023,"finding":"Proximity biotinylation (BioID) identified 28 GEF-dependent RAB18 interactors. Novel interactions with SEC22A, TMCO4, and INPP5B were directly validated. RAB18 interacts with the sterol isomerase EBP and lipid transporter ORP2/OSBPL2. RAB18 loss or dysregulation causes accumulation of the cholesterol precursor lathosterol (EBP product) and impairs de novo cholesterol biosynthesis, implicating RAB18 in sterol mobilization at membrane contact sites.","method":"Proximity biotinylation (BioID), validation co-IPs, cholesterol precursor quantification by mass spectrometry in RAB18-null HeLa cells and RAB3GAP1-null patient fibroblasts, de novo cholesterol biosynthesis assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — proximity biotinylation with direct biochemical validation, metabolic assays in two independent cell models (KO cell line + patient fibroblasts), multiple orthogonal methods","pmids":["37774976"],"is_preprint":false},{"year":2019,"finding":"Rab18 plays a role upstream of adipose triglyceride lipase (ATGL) in lipolysis; dominant-negative GDP-locked Rab18-S22N causes loss of GFP-ATGL and Arf4-GFP from LD surfaces, similar to brefeldin A treatment targeting GBF1/Arf exchange factors. Arf4-GFP accumulates on Rab18-positive LDs, suggesting Rab18 recruits ATGL via the Arf/GBF1 machinery.","method":"Dominant-negative Rab18-S22N expression, GFP-ATGL and Arf4-GFP localization assays, brefeldin A treatment, live-cell imaging","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — functional localization assays with dominant-negative mutant and pharmacological tool, single lab, single method type","pmids":["31610914"],"is_preprint":false},{"year":2024,"finding":"2'3'-cGAMP binds Rab18 directly and facilitates GTP loading, thereby activating Rab18. Activated Rab18 promotes FosB transcription to facilitate cell migration. Lovastatin-induced deprenylation of Rab18 abolishes 2'3'-cGAMP recognition and suppresses cell migration, establishing a cGAS/cGAMP/Rab18/FosB signaling axis in cell migration independent of innate immunity.","method":"2'3'-cGAMP interactome/pulldown, GTP loading assays, FosB transcription reporter, Rab18 knockdown/overexpression, migration assays, lovastatin treatment","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding assay showing nucleotide loading effect plus functional migration readouts, single lab","pmids":["39413198"],"is_preprint":false},{"year":2023,"finding":"The small GTPase RAB18 recruits the latent CSNCSN7A-CRL3 complex to lipid droplets during adipogenesis. CRL3 is activated by neddylation at LDs, and this is an essential event for LD formation during adipogenesis. Knockdown of CSN7A or RAB18 blocks adipogenesis.","method":"Co-immunoprecipitation, siRNA knockdown of RAB18 and CSN7A, adipogenesis assays, neddylation inhibition","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus knockdown with functional adipogenesis phenotype, single lab","pmids":["37091236"],"is_preprint":false},{"year":2024,"finding":"PRRSV-2 infection transcriptionally upregulates RAB18 through RIG-I/MAVS-mediated canonical NF-κB activation. RAB18 promotes chaperone-mediated autophagy (CMA)-mediated lipolysis; during PRRSV-2 infection, enhanced associations are detected between RAB18 and PLIN2, LAMP2A, and HSPA8/HSC70. The C-terminal domain of HSPA8 binds the switch II domain of RAB18, and the CTD of PLIN2 associates with HSPA8, indicating HSPA8 bridges RAB18 and PLIN2 in CMA-mediated lipolysis.","method":"shRNA screen, co-immunoprecipitation of RAB18 with PLIN2/LAMP2A/HSPA8, domain mapping by pulldown, NF-κB pathway inhibition, LAMP2A/HSPA8 knockdown with viral yield readout","journal":"PLoS pathogens","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP with domain mapping plus NF-κB pathway connection and functional viral/lipolysis readouts, single lab","pmids":["38607975"],"is_preprint":false},{"year":2025,"finding":"Rab18 in its GTP-bound state anchors to membranes at basal bodies in Chlamydomonas and recruits the BBSome as an effector to facilitate BBSome diffusion through the transition zone for ciliary entry, thereby maintaining ciliary homeostasis of signaling proteins (including phospholipase D).","method":"Localization studies in Chlamydomonas, dominant GTPase state mutants, BBSome co-localization and functional ciliary entry assays","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 / Weak — preprint, Chlamydomonas model, localization and mutant studies without biochemical reconstitution; functional relevance to mammalian RAB18 uncertain","pmids":["bio_10.1101_2025.02.04.636545"],"is_preprint":true}],"current_model":"RAB18 is a small GTPase that cycles between GDP-bound (inactive) and GTP-bound (active, membrane-anchored) states, regulated by the RAB3GAP1/2 heterodimeric GEF complex (and also TRAPPII) and the TBC1D20 GAP; active RAB18 localizes primarily to the ER and the monolayer surface of lipid droplets, where it tethers ER to LDs by binding the NRZ tethering complex (NAG-RINT1-ZW10) and associated SNAREs (Syntaxin18, Use1, BNIP1), thereby promoting LD growth and lipid transfer; it also recruits ATGL (via Arf/GBF1 machinery) and ACSL3 to LDs to regulate lipolysis and fatty acid activation, interacts with PLIN2 and HSPA8 in CMA-mediated lipolysis, modulates autophagy/autolysosome maturation through Atg6/Beclin1 and Vps34 Complex I, controls ER morphology and ER-Golgi retrograde trafficking, regulates neuronal migration by preventing lysosomal degradation of N-cadherin, controls focal adhesion dynamics by interacting with kinectin-1 to drive kinesin-1-dependent ER anterograde transport, inhibits secretory granule exocytosis in neuroendocrine cells, and is co-opted by multiple RNA viruses (HCV, DENV, CSFV, BKPyV) through direct binding of viral nonstructural proteins to facilitate viral replication and assembly at LD-ER interfaces."},"narrative":{"mechanistic_narrative":"RAB18 is a small GTPase that cycles between an inactive GDP-bound state and an active, membrane-anchored GTP-bound state to coordinate lipid droplet (LD) biology, ER architecture, and membrane trafficking [PMID:15914536, PMID:21473985]. Its activation requires the RAB3GAP1/RAB3GAP2 heterodimeric GEF, which targets RAB18 to the ER and is necessary for proper ER tubule/sheet morphology, while the TRAPPII complex provides a second GEF activity that drives COPI-dependent recruitment of RAB18 to LD surfaces; the GAP TBC1D20 cycles RAB18 off ER membranes [PMID:24891604, PMID:28003315, PMID:26063829]. Active RAB18 localizes to the ER and the phospholipid monolayer of LDs [PMID:16207721], where it controls LD growth by tethering ER to LDs through binding the ER-associated NRZ (NAG-RINT1-ZW10) complex and its associated SNAREs (Syntaxin18, Use1, BNIP1), so that loss of RAB18 reduces mature LD numbers and lipid storage and elevates ER stress [PMID:29367353]. At the LD it scaffolds lipid-metabolic machinery — interacting with PLIN2 and ACSL3 in a tripartite complex, recruiting ATGL via the Arf/GBF1 system, and bridging to HSPA8/PLIN2 in chaperone-mediated lipolysis — thereby governing triacylglycerol storage and lipolysis in adipocytes, hepatic stellate cells, and myoblasts [PMID:33713834, PMID:31610914, PMID:38607975]. RAB18 also functions in sterol mobilization at membrane contact sites, where its loss perturbs cholesterol precursor handling involving EBP and ORP2 [PMID:37774976], in ER-to-Golgi retrograde trafficking and Golgi integrity [PMID:18664496], and in autophagy/lysosomal maturation through RAB3GAP-dependent engagement of the Vps34/Beclin1 complex and crosstalk with RAB7 [PMID:28342870, PMID:32248620, PMID:30721447]. In neurons it supports radial migration by limiting lysosomal degradation of N-cadherin [PMID:26879639], and it drives focal adhesion maturation and directional migration via kinectin-1-dependent anterograde ER transport [PMID:32525992]. Loss-of-function mutations in RAB18 that abolish guanine-nucleotide binding cause Warburg Micro syndrome [PMID:21473985]. RAB18 is co-opted by multiple viruses, binding nonstructural proteins of HCV, DENV, and CSFV in a GTP-dependent manner and acting as a host factor for BKPyV, to support replication and assembly at LD-ER interfaces [PMID:23935497, PMID:24696471, PMID:32419589, PMID:28815213].","teleology":[{"year":2005,"claim":"Establishing where RAB18 acts: the initial question was the subcellular site and nucleotide-state dependence of RAB18 function, answered by showing it decorates the LD monolayer and drives ER-LD apposition.","evidence":"LD proteomics, immunoelectron microscopy, and dominant-active/negative mutants in HepG2 cells and adipocytes","pmids":["15914536","16207721"],"confidence":"High","gaps":["Did not identify the tethering machinery linking ER to LDs","Did not establish the GEF/GAP regulating RAB18 activation"]},{"year":2007,"claim":"Extending RAB18 beyond LDs: tested whether RAB18 had roles in regulated secretion, showing activation-dependent association with secretory granules that inhibits exocytosis.","evidence":"Immunoelectron microscopy and secretion assays with GTPase mutants in PC12 and AtT20 neuroendocrine cells","pmids":["17488286"],"confidence":"High","gaps":["Effector mediating secretory inhibition not identified","Relationship to LD function unresolved"]},{"year":2008,"claim":"Asked whether RAB18 controls early secretory membrane trafficking, revealing a role in Golgi integrity and selective COPI-independent Golgi-to-ER retrograde transport.","evidence":"siRNA, dominant-negative mutants, and live-cell cargo transport/photobleach assays","pmids":["18664496"],"confidence":"High","gaps":["Tethers/SNAREs at the Golgi-ER step not defined","How this relates to LD function unclear"]},{"year":2011,"claim":"Connected RAB18 to human disease and to insulin signaling: nucleotide-null mutations cause Warburg Micro syndrome, and PI3K-dependent recruitment links RAB18 to lipogenesis as well as lipolysis.","evidence":"Autozygosity mapping with nucleotide-binding assays and zebrafish knockdown; PI3K inhibition with gain/loss-of-function in 3T3-L1 adipocytes","pmids":["21473985","21829560"],"confidence":"High","gaps":["Molecular cause of the developmental/disease phenotype not mechanistically resolved","How a single GTPase mediates both lipogenesis and lipolysis unexplained"]},{"year":2014,"claim":"Identified the activation machinery: RAB3GAP1/2 is the RAB18 GEF that controls ER targeting and ER morphology, linking disease mutations in the GEF to loss of RAB18 activation.","evidence":"Direct GEF activity assay, fractionation, ER-marker imaging, and disease-mutant analysis","pmids":["24891604"],"confidence":"High","gaps":["Did not address whether other GEFs act on RAB18 at the LD","Downstream effectors of activated RAB18 not defined"]},{"year":2014,"claim":"Established RAB18 as a virally hijacked host factor: GTP-dependent binding to HCV NS5A and DENV-related FASN recruitment route replicase/lipid machinery to LD-ER sites.","evidence":"Reciprocal co-IP with GTP/GDP-locked mutants, colocalization, and knockdown/overexpression with viral output in HCV- and DENV-infected cells","pmids":["23935497","24696471","24997429"],"confidence":"High","gaps":["Whether virus exploits native ER-LD tethering function not directly tested","Structural basis of NS5A binding unresolved"]},{"year":2015,"claim":"Completed the GTPase cycle and broadened physiology: TBC1D20 is the RAB18 GAP that cycles it off the ER, and RAB18 activity/prenylation drives LD loss during hepatic stellate cell activation.","evidence":"In vitro GAP assay with fractionation in TBC1D20-null cells; functional GTPase/prenylation mutants and knockdown in hepatic stellate cells","pmids":["26063829","25482505"],"confidence":"Medium","gaps":["GAP activity was modest in vitro","Whether stellate-cell LD loss uses tethering or lipolytic effectors not defined"]},{"year":2016,"claim":"Resolved how RAB18 reaches LDs and acts in neurons: TRAPPII (delivered via COPI) is a second GEF activating RAB18 at LDs, and in cortex RAB18 supports radial migration by protecting surface N-cadherin from lysosomal degradation.","evidence":"GEF assay with CRISPR/siRNA and LD morphology; in utero electroporation with N-cadherin and lysosomal-inhibitor follow-up","pmids":["28003315","26879639"],"confidence":"Medium","gaps":["How two GEFs (RAB3GAP vs TRAPPII) partition RAB18 between ER and LD pools unclear","Mechanism by which RAB18 limits N-cadherin degradation undefined"]},{"year":2017,"claim":"Linked RAB18 to autophagy/proteostasis and a further virus: RAB18 promotes macroautophagy via its RAB3GAP GEFs, and it is an essential host factor for BKPyV retrograde transport with the NRZ complex.","evidence":"Autophagy flux assays with RAB3GAP epistasis in fibroblasts; genome-wide siRNA screen with validation in renal epithelial cells","pmids":["28342870","28815213"],"confidence":"Medium","gaps":["Direct autophagy effectors of RAB18 not yet identified","Whether BKPyV transport uses the same NRZ tether as LDs not directly shown"]},{"year":2018,"claim":"Delivered the central mechanism: activated RAB18 tethers ER to LDs by binding the NRZ complex and its SNAREs, defining how RAB18 drives LD growth — though one cell type showed only modest LD phenotypes, indicating context dependence.","evidence":"Co-IP/pulldown of RAB18-NRZ-SNARE complex and CRISPR/siRNA with LD/ER-stress readouts; contrasting CRISPR knockout in mammary carcinoma cells","pmids":["29367353","29949452"],"confidence":"High","gaps":["Basis of cell-type-specific dependence on RAB18 for LDs unresolved","How tethering is coupled to lipid transfer not directly demonstrated"]},{"year":2019,"claim":"Mapped RAB18's lipolytic and autophagic outputs: it acts upstream of ATGL via Arf/GBF1, supplies LD-derived lipids for autophagosome formation (triggering ATG9A-based compensation), and partners with RAB7 in neuronal lysosomal transport.","evidence":"Dominant-negative localization assays for ATGL/Arf4; RAB18 knockout with lipidomics/phosphoproteomics; co-IP/fractionation and KO neuron phenotypes with RAB7","pmids":["31610914","31874152","30721447"],"confidence":"Medium","gaps":["Direct vs indirect nature of RAB18-RAB7 relationship unclear","ATGL recruitment mechanism inferred from dominant-negative phenotype only"]},{"year":2020,"claim":"Expanded the effector repertoire: GTP-RAB18 binds Atg6/Beclin1 to promote autolysosome maturation via Vps34 Complex I, interacts with kinectin-1 to drive kinesin-1-dependent ER transport for focal adhesion maturation, and binds CSFV NS5A.","evidence":"GTP-state co-IPs and Drosophila epistasis; co-IP with kinectin-1, TIRF FA imaging and chemotaxis; co-IP/GST-pulldown with viral knockdown","pmids":["32248620","32525992","32419589"],"confidence":"Medium","gaps":["Whether these effector interactions occur simultaneously or in distinct RAB18 pools unknown","Beclin1 finding from Drosophila not confirmed in mammals"]},{"year":2021,"claim":"Defined the LD-resident effector scaffold: RAB18 forms a PLIN2-ACSL3 complex on LDs, with PLIN2 required for RAB18 LD localization and RAB18 promoting ACSL3 recruitment, controlling TAG storage.","evidence":"Co-IP, PLIN2/ACSL3 knockdown, and TAG/LD morphology readouts in C2C12 myoblasts","pmids":["33713834"],"confidence":"Medium","gaps":["Order of assembly of the RAB18-PLIN2-ACSL3 complex unresolved","Whether ACSL3 fatty-acid activation is directly regulated not shown"]},{"year":2023,"claim":"Broadened RAB18 into sterol biology and ubiquitin signaling: GEF-dependent interactomics linked RAB18 to EBP/ORP2-dependent cholesterol biosynthesis at contact sites, and RAB18 recruits the CSN7A-CRL3 complex to LDs to drive adipogenesis.","evidence":"BioID with validation co-IPs and cholesterol-precursor mass spectrometry in KO cells and patient fibroblasts; co-IP and knockdown with adipogenesis/neddylation assays","pmids":["37774976","37091236"],"confidence":"High","gaps":["Mechanism by which RAB18 promotes sterol transfer not biochemically reconstituted","Substrate of LD-localized CRL3 not identified"]},{"year":2024,"claim":"Identified a novel activation cue and a CMA function: 2'3'-cGAMP binds RAB18 and promotes GTP loading to drive a FosB migration axis, while RAB18 bridges HSPA8 and PLIN2 in chaperone-mediated lipolysis upregulated during PRRSV-2 infection.","evidence":"cGAMP pulldown/GTP-loading assays with FosB reporter and migration assays; co-IP/domain mapping with HSPA8/PLIN2/LAMP2A and NF-kB pathway dissection","pmids":["39413198","38607975"],"confidence":"Medium","gaps":["Physiological significance of cGAMP-driven RAB18 activation outside migration unclear","Whether CMA-lipolysis function is conserved beyond infection context untested"]},{"year":2025,"claim":"Probed a ciliary role: GTP-RAB18 at basal bodies recruits the BBSome for ciliary entry in Chlamydomonas, suggesting a trafficking function in cilia.","evidence":"Localization and GTPase-state mutants with BBSome colocalization/ciliary-entry assays in Chlamydomonas (preprint)","pmids":["bio_10.1101_2025.02.04.636545"],"confidence":"Low","gaps":["Preprint not peer-reviewed and without biochemical reconstitution","Relevance of this ciliary role to mammalian RAB18 unestablished"]},{"year":null,"claim":"How the many RAB18 functions (ER-LD tethering, lipolysis, sterol transfer, autophagy, migration, viral hijacking) are partitioned across distinct activation cues, GEFs, and effector pools within a cell remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model defining how RAB18 selects among NRZ, PLIN2/ACSL3, kinectin-1, Beclin1, and EBP/ORP2 effectors","Mechanism coupling RAB18-mediated tethering to actual lipid/sterol transfer not reconstituted","Determinants directing RAB18 between RAB3GAP/ER and TRAPPII/LD activation undefined"]}],"mechanism_profile":{"molecular_activity":[],"localization":[{"term_id":"GO:0005811","term_label":"lipid droplet","supporting_discovery_ids":[0,1,16,23]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[7,16,21]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[3]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[19]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[16,23,24]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[14,18,20]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[3,21]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[4]}],"complexes":["RAB18-PLIN2-ACSL3 complex"],"partners":["RAB3GAP1","RAB3GAP2","TBC1D20","PLIN2","ACSL3","KTN1","HSPA8","RAB7"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NP72","full_name":"Ras-related protein Rab-18","aliases":[],"length_aa":206,"mass_kda":23.0,"function":"The small GTPases Rab are key regulators of intracellular membrane trafficking, from the formation of transport vesicles to their fusion with membranes (PubMed:24891604, PubMed:30970241). Rabs cycle between an inactive GDP-bound form and an active GTP-bound form that is able to recruit to membranes different sets of downstream effectors directly responsible for vesicle formation, movement, tethering and fusion (PubMed:24891604, PubMed:30970241). RAB18 is required for the localization of ZFYVE1 to lipid droplets and for its function in mediating the formation of endoplasmic reticulum-lipid droplets (ER-LD) contacts (PubMed:30970241). Also required for maintaining endoplasmic reticulum structure (PubMed:24891604). Plays a role in apical endocytosis/recycling (By similarity). Plays a key role in eye and brain development and neurodegeneration (PubMed:21473985)","subcellular_location":"Endoplasmic reticulum membrane; Golgi apparatus, cis-Golgi network membrane; Lipid droplet; Apical cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9NP72/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/RAB18","classification":"Not Classified","n_dependent_lines":182,"n_total_lines":1208,"dependency_fraction":0.15066225165562913},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000099246","cell_line_id":"CID000421","localizations":[{"compartment":"vesicles","grade":3},{"compartment":"er","grade":1}],"interactors":[{"gene":"COPB2","stoichiometry":0.2},{"gene":"COPE","stoichiometry":0.2},{"gene":"GDI1","stoichiometry":0.2},{"gene":"GDI2","stoichiometry":0.2},{"gene":"GORASP2","stoichiometry":0.2},{"gene":"BET1L","stoichiometry":0.2},{"gene":"PATZ1","stoichiometry":0.2},{"gene":"SRFBP1","stoichiometry":0.2},{"gene":"RTN4","stoichiometry":0.2},{"gene":"SEC61B","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID000421","total_profiled":1310},"omim":[{"mim_id":"621555","title":"TRANSMEMBRANE AND COILED-COIL DOMAINS PROTEIN 4; TMCO4","url":"https://www.omim.org/entry/621555"},{"mim_id":"619420","title":"MARTSOLF SYNDROME 2; MARTS2","url":"https://www.omim.org/entry/619420"},{"mim_id":"617503","title":"DENN DOMAIN-CONTAINING PROTEIN 3; DENND3","url":"https://www.omim.org/entry/617503"},{"mim_id":"615663","title":"WARBURG MICRO SYNDROME 4; WARBM4","url":"https://www.omim.org/entry/615663"},{"mim_id":"614225","title":"WARBURG MICRO SYNDROME 2; WARBM2","url":"https://www.omim.org/entry/614225"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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Rab18 overexpression induces close apposition of LDs to rough ER-derived membrane cisternae (termed LD-associated membrane, LAM), and this is linked to a decrease in ADRP on LDs.\",\n      \"method\": \"Proteomic analysis of LD fraction, immunohistochemistry, western blotting, dominant-active/dominant-negative mutant expression, RNA interference, brefeldin A treatment\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (proteomics, IHC, dominant mutants, RNAi, BFA), replicated across cell lines\",\n      \"pmids\": [\"15914536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"GFP-Rab18 localizes directly to the phospholipid monolayer surface of LDs (shown by immunoelectron microscopy), and this association is increased upon lipolytic stimulation of adipocytes by beta-adrenergic agonists. A caveolin-3 truncation mutant that inhibits LD catabolism labels mutually exclusive LD subpopulations from Rab18-labeled LDs.\",\n      \"method\": \"GFP tagging, immunoelectron microscopy, lipolytic stimulation, beta-adrenergic antagonist reversal, co-expression of caveolin-3 mutant\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — immunoelectron microscopy directly demonstrates monolayer surface association; functional regulation by lipolytic stimulation shown with antagonist reversal\",\n      \"pmids\": [\"16207721\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"In neuroendocrine cells (PC12 and AtT20), Rab18 associates with a subpopulation of secretory granules upon stimulation of the regulated secretory pathway, and this association is linked to inhibition of secretory activity. Dominant-active Rab18 associates with secretory granules; dominant-inactive Rab18 remains cytosolic.\",\n      \"method\": \"Immunofluorescence, immunoelectron microscopy, dominant-active/dominant-negative mutant expression, secretion assays in PC12 and AtT20 cells\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — immunoelectron microscopy plus functional secretion assays with GTPase mutants in two cell lines\",\n      \"pmids\": [\"17488286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Rab18 overexpression or siRNA knockdown severely disrupts the Golgi complex and reduces secretion of VSVG. Rab18 mutants specifically enhance retrograde COPI-independent Golgi-to-ER transport of beta-1,4-galactosyltransferase-YFP but not COPI-dependent p58-YFP cargo. Rab18-S22N potentiates BFA-induced ER-Golgi fusion, indicating a role for Rab18 in ER-Golgi trafficking.\",\n      \"method\": \"siRNA knockdown, dominant-negative mutant expression, VSVG-GFP secretion assay, photobleach assay for retrograde Golgi-ER transport, BFA treatment\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (siRNA, dominant mutants, live-cell transport assays) showing Golgi disruption and selective retrograde trafficking defects\",\n      \"pmids\": [\"18664496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Loss-of-function mutations in RAB18 (Leu24Gln and Arg93del) cause Warburg Micro syndrome. Nucleotide binding assays showed these mutant proteins are functionally null — unable to bind guanine nucleotides. Knockdown of rab18 in zebrafish confirmed a conserved developmental role.\",\n      \"method\": \"Autozygosity mapping, Sanger sequencing, MLPA, nucleotide binding assays, zebrafish rab18 knockdown\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — direct biochemical assay (nucleotide binding) demonstrating functional null mutations, combined with genetic mapping and in vivo knockdown\",\n      \"pmids\": [\"21473985\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Insulin induces Rab18 recruitment to LD surfaces via phosphatidylinositol 3-kinase (PI3K) signaling in 3T3-L1 adipocytes. Rab18 overexpression increases basal lipogenesis, and Rab18 silencing impairs the lipogenic response to insulin, establishing Rab18 as a mediator of insulin-stimulated lipogenesis as well as beta-adrenergic lipolysis.\",\n      \"method\": \"PI3K inhibitor treatment, Rab18 overexpression, siRNA silencing, lipogenesis assays in 3T3-L1 adipocytes\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — PI3K inhibitor and genetic gain/loss-of-function in adipocytes with functional lipogenesis readout, single lab\",\n      \"pmids\": [\"21829560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Rab18 binds to the HCV nonstructural protein NS5A; GTP-bound (active) Rab18 binds NS5A more strongly than constitutively GDP-bound Rab18. Rab18 promotes physical association of NS5A and other replicase components with LDs in HCV-infected cells and modulates HCV genome replication and production of infectious virions.\",\n      \"method\": \"Unbiased proteomics, co-immunoprecipitation, GTP/GDP-locked Rab18 mutant binding assays, colocalization studies in HCV-infected cells, Rab18 knockdown/overexpression with viral output measurement\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP with GTPase state-dependent binding, colocalization, and functional knockdown/overexpression with viral phenotype\",\n      \"pmids\": [\"23935497\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The Rab3GAP complex (RAB3GAP1/RAB3GAP2 heterodimer) is a specific guanine nucleotide exchange factor (GEF) for Rab18. Rab3GAP localizes to the ER and is necessary and sufficient for ER targeting of Rab18. In the absence of Rab3GAP or Rab18 function, ER tubular networks (marked by reticulon 4) are disrupted and ER sheets (CLIMP-63) spread into the cell periphery. Disease-associated point mutations in Rab3GAP1 (T18P, E24V) and Rab3GAP2 (R426C) abolish GEF and membrane-targeting activities.\",\n      \"method\": \"GEF activity assay, subcellular fractionation, immunofluorescence with ER markers, dominant-negative and disease mutants, siRNA knockdown, Rab18 membrane recruitment assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — direct GEF activity assay plus multiple orthogonal functional readouts (ER morphology, membrane targeting) and disease mutant validation\",\n      \"pmids\": [\"24891604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Active (GTP-bound) Rab18 is required for targeting of fatty acid synthase (FASN) to ER and LD sites of dengue virus (DENV) replication. The interaction of FASN with DENV NS3 protein depends on conversion of Rab18 from GDP- to GTP-bound form. Inactive Rab18 or Rab18 knockdown reduces DENV replication, viral protein/RNA production, and LD induction.\",\n      \"method\": \"GTP/GDP-locked Rab18 mutant expression, siRNA knockdown, colocalization, co-immunoprecipitation, viral output measurement (RNA, protein, progeny)\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP, GTPase state-dependent FASN recruitment, functional knockdown and mutant overexpression with viral phenotype, multiple readouts\",\n      \"pmids\": [\"24696471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Rab18 is required for trafficking of the HCV core protein to LDs for viral assembly. Rab18 knockdown reduces intracellular and extracellular viral infectivity but not intracellular viral replication, and blocks recruitment of the HCV core protein to LDs. Rab18 overexpression increases extracellular viral infectivity ~2-fold.\",\n      \"method\": \"Rab18 siRNA knockdown, Rab18 overexpression, colocalization of Rab18 with core protein, viral infectivity assays\",\n      \"journal\": \"Virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — complementary knockdown/overexpression with functional readouts and colocalization, single lab\",\n      \"pmids\": [\"24997429\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"TBC1D20 shows RAB18 GTPase-activating protein (GAP) activity in vitro and is a physiological substrate regulator of RAB18. In cells lacking TBC1D20, RAB18 becomes more stably ER-associated and less cytosolic, indicating TBC1D20 promotes RAB18 cycling off the ER membrane. This establishes RAB18 as a substrate of TBC1D20.\",\n      \"method\": \"In vitro GAP activity assay, subcellular fractionation, immunofluorescence of RAB18 localization in TBC1D20-null cells\",\n      \"journal\": \"Open biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro GAP assay plus cellular localization data, but modest GAP activity noted in vitro; single lab\",\n      \"pmids\": [\"26063829\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In hepatic stellate cells, Rab18 GTPase activity and isoprenylation are required for stellate cell LD loss and activation marker induction. Rab18 knockdown retards LD loss and blocks activation; Rab18 mRNA and protein increase during activation. Rab18 is identified as a retinoic acid-responsive, LD-associated protein in HSCs.\",\n      \"method\": \"siRNA knockdown, plasmid overexpression, functional mutants (GTPase-dead and isoprenylation-defective), gene array, immunocytochemistry, in vivo liver injury model\",\n      \"journal\": \"Hepatology (Baltimore, Md.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional mutants and knockdown with cellular phenotype readout in primary and transformed cells, single lab\",\n      \"pmids\": [\"25482505\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Mammalian TRAPPII complex acts as a GEF for both Rab18 and Rab1. Inactivation of TRAPPII-specific subunits (siRNA or CRISPR-Cas9 deletion) impairs Rab18 recruitment to LD surfaces and results in aberrantly large LDs with reduced lipolysis. The COPI-TRAPPII interaction is required for Rab18 recruitment to LDs, suggesting COPI brings TRAPPII to the LD surface where TRAPPII activates Rab18.\",\n      \"method\": \"GEF activity assay, siRNA depletion, CRISPR-Cas9 deletion, LD morphology analysis, Rab18 localization assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — direct GEF assay combined with CRISPR knockout and siRNA, multiple orthogonal methods demonstrating Rab18 activation and LD recruitment\",\n      \"pmids\": [\"28003315\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In developing mouse cerebral cortex, RAB18 regulates neuronal radial migration and morphogenesis. Suppression of RAB18 by in utero electroporation, or overexpression of dominant-negative RAB18, impairs radial migration. Loss of RAB18 accelerates N-cadherin degradation via the lysosomal pathway, reducing surface N-cadherin levels on neurons.\",\n      \"method\": \"In utero electroporation (shRNA knockdown and dominant-negative overexpression), in vitro neurite growth assays, N-cadherin surface level measurement, lysosomal inhibitor treatment\",\n      \"journal\": \"Molecular brain\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo electroporation with migration phenotype and mechanistic follow-up showing N-cadherin lysosomal degradation, single lab\",\n      \"pmids\": [\"26879639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"RAB18 positively modulates macroautophagy in primary human fibroblasts; knockdown reduces autophagic activity while overexpression enhances it. This function depends on RAB3GAP1 and RAB3GAP2 as GEFs. RAB18 knockdown also impairs proteostasis, causing accumulation of ubiquitinated proteins.\",\n      \"method\": \"siRNA knockdown, overexpression, autophagy flux assays, ubiquitinated protein accumulation measurement, RAB3GAP1/2 co-knockdown epistasis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with RAB3GAP1/2 plus gain/loss-of-function autophagy assays, single lab\",\n      \"pmids\": [\"28342870\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Rab18 is required for BK polyomavirus (BKPyV) infection; genome-wide siRNA screen and follow-up identified Rab18 and syntaxin 18 as essential host factors. Rab18 and the NRZ complex function in retrograde transport of BKPyV from late endosomes to the ER lumen.\",\n      \"method\": \"Whole-genome siRNA screen, siRNA knockdown validation, viral infection assays in primary human renal proximal tubule epithelial cells\",\n      \"journal\": \"mSphere\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genome-wide screen followed by targeted validation, functional viral infectivity readout, single lab\",\n      \"pmids\": [\"28815213\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Rab18 controls LD growth and maturation by tethering the ER to LDs. LD-associated Rab18 (activated by its GEF Rab3GAP1/2) binds specifically to the ER-associated NRZ (NAG-RINT1-ZW10) tethering complex and associated SNAREs (Syntaxin18, Use1, BNIP1), recruiting ER to LDs and forming direct ER-LD contacts. Rab18 deficiency or NRZ/SNARE defects cause drastically reduced mature LD numbers, decreased lipid storage, and increased ER stress.\",\n      \"method\": \"Co-immunoprecipitation, pulldown assays, CRISPR/siRNA knockdown of Rab18 and NRZ/SNARE components, LD morphology and lipid quantification, ER stress markers\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — biochemical pulldown/co-IP identifying the Rab18-NRZ-SNARE complex plus functional rescue/knockdown experiments with multiple orthogonal readouts\",\n      \"pmids\": [\"29367353\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In human mammary carcinoma cells, Rab18 localizes to the ER and LD membranes in an activation-state-dependent manner, but knockout of Rab18 causes only modest reductions in LD size and numbers with no evidence for essential roles in LD formation, LD turnover on starvation, or targeting of several LD proteins.\",\n      \"method\": \"Rab18 knockout (CRISPR), LD morphology quantification, protein targeting assays, starvation-induced LD turnover\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR knockout with multiple negative functional readouts; this represents a cell-type-specific negative result in mammary carcinoma cells\",\n      \"pmids\": [\"29949452\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"RAB18 knockout impairs lipid droplet catabolism and fatty acid release, reducing lipid availability for autophagosome formation. Compensatory adaptations include increased ATG2B expression and phosphorylation, increased ATG12-ATG5 conjugate formation, and enhanced ATG9A phosphorylation at Y8 and S14, which increases ATG9A trafficking. Y8 phosphorylation of ATG9A is important to maintain basal autophagy under RAB18 knockout conditions.\",\n      \"method\": \"RAB18 knockout, autophagy flux assays, lipidomics, phosphoproteomics, pharmacological inhibition of Y8 phosphorylation, ATG9A trafficking assays\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO + pharmacological inhibition with phosphoproteomic mechanistic follow-up, single lab\",\n      \"pmids\": [\"31874152\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In neurons, Rab18 associates with lysosomes, actively traffics along neurites, and colocalizes/cofractionates/coprecipitates with Rab7. Rab18-deficient neurons show impaired lysosomal transport and aberrant autophagy. Rab7 is upregulated in Rab18-deficient neurons, suggesting compensatory regulation between Rab18 and Rab7 in lysosomal/autophagic activities.\",\n      \"method\": \"Immunofluorescence, subcellular fractionation, co-immunoprecipitation, live-cell imaging of lysosomal transport, autophagosome marker assays, Rab18-/- mouse model, electron microscopy\",\n      \"journal\": \"Molecular neurobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP and fractionation plus functional KO phenotype in neurons with multiple readouts, single lab\",\n      \"pmids\": [\"30721447\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"GTP-bound Rab18 (in Drosophila) binds Atg6/Beclin1, a permanent subunit of Vps34 complexes. The Rab3GAP-Rab18 module promotes autolysosome maturation through interaction with Vps34 Complex I. Loss of Rab3GAP2 or Rab18 destabilizes Rab7-positive compartments and perturbs lysosomal biosynthetic transport. Rab3GAP2 and Rab18 are present on autophagosomal and autolysosomal membranes, colocalizing with Vps34 Complex I subunits.\",\n      \"method\": \"Co-immunoprecipitation (GTP-bound Rab18 pull-down with Atg6/Beclin1), genetic epistasis (Atg14/UVRAG overexpression phenocopying Rab3GAP-Rab18 loss), Drosophila mutant model, colocalization studies\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP with GTPase state specificity plus epistasis in Drosophila model, single lab\",\n      \"pmids\": [\"32248620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Rab18 interacts with the ER-resident protein kinectin-1 and controls anterograde kinesin-1-dependent ER transport required for maturation of nascent focal adhesions. Rab18 knockdown reduces focal adhesion size and impairs directional migration in chemotaxis. Rab18 promotes ER-focal adhesion contact formation.\",\n      \"method\": \"siRNA knockdown, live-cell TIRF microscopy of focal adhesions, co-immunoprecipitation of Rab18 with kinectin-1, ER transport assays, chemotaxis assays\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct co-IP with kinectin-1, live imaging of FA dynamics, functional migration readout, multiple orthogonal methods in single lab\",\n      \"pmids\": [\"32525992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Rab18 binds to classical swine fever virus (CSFV) NS5A protein; co-immunoprecipitation, GST-pulldown, and confocal microscopy confirm the interaction. Rab18 knockdown reduces CSFV RNA replication and virion assembly, and NS5A is redistributed in Rab18-knockdown cells.\",\n      \"method\": \"Co-immunoprecipitation, GST-pulldown, confocal microscopy, siRNA knockdown, RT-qPCR, IFA for viral protein localization\",\n      \"journal\": \"Virulence\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP plus GST-pulldown confirming direct binding, and functional knockdown/overexpression with viral readouts, single lab\",\n      \"pmids\": [\"32419589\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"LD-associated Rab18 binds to the C-terminus of perilipin 2 (PLIN2); PLIN2 depletion diminishes LD localization of Rab18. Rab18 also interacts with ACSL3 and promotes ACSL3 LD localization, forming a Rab18-PLIN2-ACSL3 complex. Loss of Rab18 reduces triacylglycerol levels and causes fewer but larger LDs in myoblast cells; overexpression increases TAG and LD number.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown of PLIN2 and ACSL3, Rab18 overexpression/knockdown, TAG quantification, LD morphology analysis in C2C12 myoblast cells\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular and cell biology of lipids\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP identifying PLIN2 and ACSL3 as binding partners with functional LD/TAG readouts, single lab\",\n      \"pmids\": [\"33713834\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Proximity biotinylation (BioID) identified 28 GEF-dependent RAB18 interactors. Novel interactions with SEC22A, TMCO4, and INPP5B were directly validated. RAB18 interacts with the sterol isomerase EBP and lipid transporter ORP2/OSBPL2. RAB18 loss or dysregulation causes accumulation of the cholesterol precursor lathosterol (EBP product) and impairs de novo cholesterol biosynthesis, implicating RAB18 in sterol mobilization at membrane contact sites.\",\n      \"method\": \"Proximity biotinylation (BioID), validation co-IPs, cholesterol precursor quantification by mass spectrometry in RAB18-null HeLa cells and RAB3GAP1-null patient fibroblasts, de novo cholesterol biosynthesis assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — proximity biotinylation with direct biochemical validation, metabolic assays in two independent cell models (KO cell line + patient fibroblasts), multiple orthogonal methods\",\n      \"pmids\": [\"37774976\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Rab18 plays a role upstream of adipose triglyceride lipase (ATGL) in lipolysis; dominant-negative GDP-locked Rab18-S22N causes loss of GFP-ATGL and Arf4-GFP from LD surfaces, similar to brefeldin A treatment targeting GBF1/Arf exchange factors. Arf4-GFP accumulates on Rab18-positive LDs, suggesting Rab18 recruits ATGL via the Arf/GBF1 machinery.\",\n      \"method\": \"Dominant-negative Rab18-S22N expression, GFP-ATGL and Arf4-GFP localization assays, brefeldin A treatment, live-cell imaging\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — functional localization assays with dominant-negative mutant and pharmacological tool, single lab, single method type\",\n      \"pmids\": [\"31610914\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"2'3'-cGAMP binds Rab18 directly and facilitates GTP loading, thereby activating Rab18. Activated Rab18 promotes FosB transcription to facilitate cell migration. Lovastatin-induced deprenylation of Rab18 abolishes 2'3'-cGAMP recognition and suppresses cell migration, establishing a cGAS/cGAMP/Rab18/FosB signaling axis in cell migration independent of innate immunity.\",\n      \"method\": \"2'3'-cGAMP interactome/pulldown, GTP loading assays, FosB transcription reporter, Rab18 knockdown/overexpression, migration assays, lovastatin treatment\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding assay showing nucleotide loading effect plus functional migration readouts, single lab\",\n      \"pmids\": [\"39413198\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The small GTPase RAB18 recruits the latent CSNCSN7A-CRL3 complex to lipid droplets during adipogenesis. CRL3 is activated by neddylation at LDs, and this is an essential event for LD formation during adipogenesis. Knockdown of CSN7A or RAB18 blocks adipogenesis.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown of RAB18 and CSN7A, adipogenesis assays, neddylation inhibition\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus knockdown with functional adipogenesis phenotype, single lab\",\n      \"pmids\": [\"37091236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PRRSV-2 infection transcriptionally upregulates RAB18 through RIG-I/MAVS-mediated canonical NF-κB activation. RAB18 promotes chaperone-mediated autophagy (CMA)-mediated lipolysis; during PRRSV-2 infection, enhanced associations are detected between RAB18 and PLIN2, LAMP2A, and HSPA8/HSC70. The C-terminal domain of HSPA8 binds the switch II domain of RAB18, and the CTD of PLIN2 associates with HSPA8, indicating HSPA8 bridges RAB18 and PLIN2 in CMA-mediated lipolysis.\",\n      \"method\": \"shRNA screen, co-immunoprecipitation of RAB18 with PLIN2/LAMP2A/HSPA8, domain mapping by pulldown, NF-κB pathway inhibition, LAMP2A/HSPA8 knockdown with viral yield readout\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP with domain mapping plus NF-κB pathway connection and functional viral/lipolysis readouts, single lab\",\n      \"pmids\": [\"38607975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Rab18 in its GTP-bound state anchors to membranes at basal bodies in Chlamydomonas and recruits the BBSome as an effector to facilitate BBSome diffusion through the transition zone for ciliary entry, thereby maintaining ciliary homeostasis of signaling proteins (including phospholipase D).\",\n      \"method\": \"Localization studies in Chlamydomonas, dominant GTPase state mutants, BBSome co-localization and functional ciliary entry assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — preprint, Chlamydomonas model, localization and mutant studies without biochemical reconstitution; functional relevance to mammalian RAB18 uncertain\",\n      \"pmids\": [\"bio_10.1101_2025.02.04.636545\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"RAB18 is a small GTPase that cycles between GDP-bound (inactive) and GTP-bound (active, membrane-anchored) states, regulated by the RAB3GAP1/2 heterodimeric GEF complex (and also TRAPPII) and the TBC1D20 GAP; active RAB18 localizes primarily to the ER and the monolayer surface of lipid droplets, where it tethers ER to LDs by binding the NRZ tethering complex (NAG-RINT1-ZW10) and associated SNAREs (Syntaxin18, Use1, BNIP1), thereby promoting LD growth and lipid transfer; it also recruits ATGL (via Arf/GBF1 machinery) and ACSL3 to LDs to regulate lipolysis and fatty acid activation, interacts with PLIN2 and HSPA8 in CMA-mediated lipolysis, modulates autophagy/autolysosome maturation through Atg6/Beclin1 and Vps34 Complex I, controls ER morphology and ER-Golgi retrograde trafficking, regulates neuronal migration by preventing lysosomal degradation of N-cadherin, controls focal adhesion dynamics by interacting with kinectin-1 to drive kinesin-1-dependent ER anterograde transport, inhibits secretory granule exocytosis in neuroendocrine cells, and is co-opted by multiple RNA viruses (HCV, DENV, CSFV, BKPyV) through direct binding of viral nonstructural proteins to facilitate viral replication and assembly at LD-ER interfaces.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"RAB18 is a small GTPase that cycles between an inactive GDP-bound state and an active, membrane-anchored GTP-bound state to coordinate lipid droplet (LD) biology, ER architecture, and membrane trafficking [#0, #4]. Its activation requires the RAB3GAP1/RAB3GAP2 heterodimeric GEF, which targets RAB18 to the ER and is necessary for proper ER tubule/sheet morphology, while the TRAPPII complex provides a second GEF activity that drives COPI-dependent recruitment of RAB18 to LD surfaces; the GAP TBC1D20 cycles RAB18 off ER membranes [#7, #12, #10]. Active RAB18 localizes to the ER and the phospholipid monolayer of LDs [#1], where it controls LD growth by tethering ER to LDs through binding the ER-associated NRZ (NAG-RINT1-ZW10) complex and its associated SNAREs (Syntaxin18, Use1, BNIP1), so that loss of RAB18 reduces mature LD numbers and lipid storage and elevates ER stress [#16]. At the LD it scaffolds lipid-metabolic machinery — interacting with PLIN2 and ACSL3 in a tripartite complex, recruiting ATGL via the Arf/GBF1 system, and bridging to HSPA8/PLIN2 in chaperone-mediated lipolysis — thereby governing triacylglycerol storage and lipolysis in adipocytes, hepatic stellate cells, and myoblasts [#23, #25, #28]. RAB18 also functions in sterol mobilization at membrane contact sites, where its loss perturbs cholesterol precursor handling involving EBP and ORP2 [#24], in ER-to-Golgi retrograde trafficking and Golgi integrity [#3], and in autophagy/lysosomal maturation through RAB3GAP-dependent engagement of the Vps34/Beclin1 complex and crosstalk with RAB7 [#14, #20, #19]. In neurons it supports radial migration by limiting lysosomal degradation of N-cadherin [#13], and it drives focal adhesion maturation and directional migration via kinectin-1-dependent anterograde ER transport [#21]. Loss-of-function mutations in RAB18 that abolish guanine-nucleotide binding cause Warburg Micro syndrome [#4]. RAB18 is co-opted by multiple viruses, binding nonstructural proteins of HCV, DENV, and CSFV in a GTP-dependent manner and acting as a host factor for BKPyV, to support replication and assembly at LD-ER interfaces [#6, #8, #22, #15].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Establishing where RAB18 acts: the initial question was the subcellular site and nucleotide-state dependence of RAB18 function, answered by showing it decorates the LD monolayer and drives ER-LD apposition.\",\n      \"evidence\": \"LD proteomics, immunoelectron microscopy, and dominant-active/negative mutants in HepG2 cells and adipocytes\",\n      \"pmids\": [\"15914536\", \"16207721\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the tethering machinery linking ER to LDs\", \"Did not establish the GEF/GAP regulating RAB18 activation\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Extending RAB18 beyond LDs: tested whether RAB18 had roles in regulated secretion, showing activation-dependent association with secretory granules that inhibits exocytosis.\",\n      \"evidence\": \"Immunoelectron microscopy and secretion assays with GTPase mutants in PC12 and AtT20 neuroendocrine cells\",\n      \"pmids\": [\"17488286\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Effector mediating secretory inhibition not identified\", \"Relationship to LD function unresolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Asked whether RAB18 controls early secretory membrane trafficking, revealing a role in Golgi integrity and selective COPI-independent Golgi-to-ER retrograde transport.\",\n      \"evidence\": \"siRNA, dominant-negative mutants, and live-cell cargo transport/photobleach assays\",\n      \"pmids\": [\"18664496\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tethers/SNAREs at the Golgi-ER step not defined\", \"How this relates to LD function unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Connected RAB18 to human disease and to insulin signaling: nucleotide-null mutations cause Warburg Micro syndrome, and PI3K-dependent recruitment links RAB18 to lipogenesis as well as lipolysis.\",\n      \"evidence\": \"Autozygosity mapping with nucleotide-binding assays and zebrafish knockdown; PI3K inhibition with gain/loss-of-function in 3T3-L1 adipocytes\",\n      \"pmids\": [\"21473985\", \"21829560\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular cause of the developmental/disease phenotype not mechanistically resolved\", \"How a single GTPase mediates both lipogenesis and lipolysis unexplained\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified the activation machinery: RAB3GAP1/2 is the RAB18 GEF that controls ER targeting and ER morphology, linking disease mutations in the GEF to loss of RAB18 activation.\",\n      \"evidence\": \"Direct GEF activity assay, fractionation, ER-marker imaging, and disease-mutant analysis\",\n      \"pmids\": [\"24891604\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address whether other GEFs act on RAB18 at the LD\", \"Downstream effectors of activated RAB18 not defined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Established RAB18 as a virally hijacked host factor: GTP-dependent binding to HCV NS5A and DENV-related FASN recruitment route replicase/lipid machinery to LD-ER sites.\",\n      \"evidence\": \"Reciprocal co-IP with GTP/GDP-locked mutants, colocalization, and knockdown/overexpression with viral output in HCV- and DENV-infected cells\",\n      \"pmids\": [\"23935497\", \"24696471\", \"24997429\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether virus exploits native ER-LD tethering function not directly tested\", \"Structural basis of NS5A binding unresolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Completed the GTPase cycle and broadened physiology: TBC1D20 is the RAB18 GAP that cycles it off the ER, and RAB18 activity/prenylation drives LD loss during hepatic stellate cell activation.\",\n      \"evidence\": \"In vitro GAP assay with fractionation in TBC1D20-null cells; functional GTPase/prenylation mutants and knockdown in hepatic stellate cells\",\n      \"pmids\": [\"26063829\", \"25482505\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"GAP activity was modest in vitro\", \"Whether stellate-cell LD loss uses tethering or lipolytic effectors not defined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Resolved how RAB18 reaches LDs and acts in neurons: TRAPPII (delivered via COPI) is a second GEF activating RAB18 at LDs, and in cortex RAB18 supports radial migration by protecting surface N-cadherin from lysosomal degradation.\",\n      \"evidence\": \"GEF assay with CRISPR/siRNA and LD morphology; in utero electroporation with N-cadherin and lysosomal-inhibitor follow-up\",\n      \"pmids\": [\"28003315\", \"26879639\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How two GEFs (RAB3GAP vs TRAPPII) partition RAB18 between ER and LD pools unclear\", \"Mechanism by which RAB18 limits N-cadherin degradation undefined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Linked RAB18 to autophagy/proteostasis and a further virus: RAB18 promotes macroautophagy via its RAB3GAP GEFs, and it is an essential host factor for BKPyV retrograde transport with the NRZ complex.\",\n      \"evidence\": \"Autophagy flux assays with RAB3GAP epistasis in fibroblasts; genome-wide siRNA screen with validation in renal epithelial cells\",\n      \"pmids\": [\"28342870\", \"28815213\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct autophagy effectors of RAB18 not yet identified\", \"Whether BKPyV transport uses the same NRZ tether as LDs not directly shown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Delivered the central mechanism: activated RAB18 tethers ER to LDs by binding the NRZ complex and its SNAREs, defining how RAB18 drives LD growth — though one cell type showed only modest LD phenotypes, indicating context dependence.\",\n      \"evidence\": \"Co-IP/pulldown of RAB18-NRZ-SNARE complex and CRISPR/siRNA with LD/ER-stress readouts; contrasting CRISPR knockout in mammary carcinoma cells\",\n      \"pmids\": [\"29367353\", \"29949452\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Basis of cell-type-specific dependence on RAB18 for LDs unresolved\", \"How tethering is coupled to lipid transfer not directly demonstrated\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Mapped RAB18's lipolytic and autophagic outputs: it acts upstream of ATGL via Arf/GBF1, supplies LD-derived lipids for autophagosome formation (triggering ATG9A-based compensation), and partners with RAB7 in neuronal lysosomal transport.\",\n      \"evidence\": \"Dominant-negative localization assays for ATGL/Arf4; RAB18 knockout with lipidomics/phosphoproteomics; co-IP/fractionation and KO neuron phenotypes with RAB7\",\n      \"pmids\": [\"31610914\", \"31874152\", \"30721447\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct vs indirect nature of RAB18-RAB7 relationship unclear\", \"ATGL recruitment mechanism inferred from dominant-negative phenotype only\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Expanded the effector repertoire: GTP-RAB18 binds Atg6/Beclin1 to promote autolysosome maturation via Vps34 Complex I, interacts with kinectin-1 to drive kinesin-1-dependent ER transport for focal adhesion maturation, and binds CSFV NS5A.\",\n      \"evidence\": \"GTP-state co-IPs and Drosophila epistasis; co-IP with kinectin-1, TIRF FA imaging and chemotaxis; co-IP/GST-pulldown with viral knockdown\",\n      \"pmids\": [\"32248620\", \"32525992\", \"32419589\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether these effector interactions occur simultaneously or in distinct RAB18 pools unknown\", \"Beclin1 finding from Drosophila not confirmed in mammals\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the LD-resident effector scaffold: RAB18 forms a PLIN2-ACSL3 complex on LDs, with PLIN2 required for RAB18 LD localization and RAB18 promoting ACSL3 recruitment, controlling TAG storage.\",\n      \"evidence\": \"Co-IP, PLIN2/ACSL3 knockdown, and TAG/LD morphology readouts in C2C12 myoblasts\",\n      \"pmids\": [\"33713834\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Order of assembly of the RAB18-PLIN2-ACSL3 complex unresolved\", \"Whether ACSL3 fatty-acid activation is directly regulated not shown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Broadened RAB18 into sterol biology and ubiquitin signaling: GEF-dependent interactomics linked RAB18 to EBP/ORP2-dependent cholesterol biosynthesis at contact sites, and RAB18 recruits the CSN7A-CRL3 complex to LDs to drive adipogenesis.\",\n      \"evidence\": \"BioID with validation co-IPs and cholesterol-precursor mass spectrometry in KO cells and patient fibroblasts; co-IP and knockdown with adipogenesis/neddylation assays\",\n      \"pmids\": [\"37774976\", \"37091236\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which RAB18 promotes sterol transfer not biochemically reconstituted\", \"Substrate of LD-localized CRL3 not identified\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identified a novel activation cue and a CMA function: 2'3'-cGAMP binds RAB18 and promotes GTP loading to drive a FosB migration axis, while RAB18 bridges HSPA8 and PLIN2 in chaperone-mediated lipolysis upregulated during PRRSV-2 infection.\",\n      \"evidence\": \"cGAMP pulldown/GTP-loading assays with FosB reporter and migration assays; co-IP/domain mapping with HSPA8/PLIN2/LAMP2A and NF-kB pathway dissection\",\n      \"pmids\": [\"39413198\", \"38607975\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological significance of cGAMP-driven RAB18 activation outside migration unclear\", \"Whether CMA-lipolysis function is conserved beyond infection context untested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Probed a ciliary role: GTP-RAB18 at basal bodies recruits the BBSome for ciliary entry in Chlamydomonas, suggesting a trafficking function in cilia.\",\n      \"evidence\": \"Localization and GTPase-state mutants with BBSome colocalization/ciliary-entry assays in Chlamydomonas (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.02.04.636545\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Preprint not peer-reviewed and without biochemical reconstitution\", \"Relevance of this ciliary role to mammalian RAB18 unestablished\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the many RAB18 functions (ER-LD tethering, lipolysis, sterol transfer, autophagy, migration, viral hijacking) are partitioned across distinct activation cues, GEFs, and effector pools within a cell remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model defining how RAB18 selects among NRZ, PLIN2/ACSL3, kinectin-1, Beclin1, and EBP/ORP2 effectors\", \"Mechanism coupling RAB18-mediated tethering to actual lipid/sterol transfer not reconstituted\", \"Determinants directing RAB18 between RAB3GAP/ER and TRAPPII/LD activation undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GTPase activity\", \"supporting_discovery_ids\": [4, 7, 10, 12]},\n      {\"term_id\": \"molecular adaptor activity\", \"supporting_discovery_ids\": [16, 23, 21]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005811\", \"supporting_discovery_ids\": [0, 1, 16, 23]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [7, 16, 21]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [19]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [16, 23, 24]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [14, 18, 20]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [3, 21]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"complexes\": [\n      \"RAB18-PLIN2-ACSL3 complex\"\n    ],\n    \"partners\": [\n      \"RAB3GAP1\",\n      \"RAB3GAP2\",\n      \"TBC1D20\",\n      \"PLIN2\",\n      \"ACSL3\",\n      \"KTN1\",\n      \"HSPA8\",\n      \"RAB7\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}